1. Field of the Invention.
The invention concerns an apparatus for producing a high-frequency induced noble gas plasma such as is used in particular in excitation in optical emission spectrometry. The excitation means employed is a high-frequency generator.
2. Description of Related Art
The noble gas considered here is helium and/or argon that shall be used at normal (atmospheric) pressure. In recent years such plasmas have assumed high significance as radiation sources in emission spectrometry. Diverse methods are known for producing the plasma. Besides plasma production by means of a DC arc (DCP), the other methods used in particular involve applying to the gas the energy required to produce the plasma in the form of high-frequency electromagnetic oscillations. A problem is incurred thereby expecially when coupling the electromagnetic power into the gas. Illustratively, the operative frequency range from 13 to 100 MHz must be selected for the generally known inductive coupling, and the power applied then is between 500 w and several kw (ICP method). If the coupling is capacitive (CMP method), a high frequency signal at 2,450 MHz is used and the power is 0.5-3 kw. In both cases, the power to be coupled therefore is exceedingly high.
A further method operating at 2,450 MHz is known, where a power of 50-200 w suffices to produce the plasma, however this method (MIP) causes difficulties in obtaining a uniformly arcing plasma when different probes are introduced. In this instance, the plasma tends to form filamentary arcing channels which strongly degrade the measurements (see for instance D. Kollotzek, Spectrochimica Acta, Vol. 37B, #2, pp 91-6, 1982).
The initially cited methods (DC arcs, ICP, CMP) are suitable for comparatively large specimens, but in view of their high performance they are initially costly. Moreover, the consumption of noble gas in such apparatus is between 5 and 20 liter//minute, which entails high operational costs. On the other hand, the above-cited MIP method is comparatively more economical in purchase cost and furthermore requires a lesser consumption of noble gases (less than 1 liter/minute). However, besides the above mentioned difficulties and lack of plasma uniformity, a further problem is encountered, namely that the plasma occasionally extinguishes and always must be re-fired externally by means of primary ions, for instance by an arc discharge.